In recent years, rapid development of information in society has brought about wide spread use of personal computers as an ordinary home appliance.
These personal computers can be used through connection with a variety of kinds of information apparatus. For example, it can transmit or receive facsimile data through connection with the telephone line and can receive a television signal through connection with a tuner for receiving the television signal.
Here, as a part of the television broadcast, the so-called teletext has been implemented by transmitting a program of the stationary image formed of characters and figures superimposed to a part of the broadcast signal.
In the teletext, the characters such as KANA and Chinese Characters of Japanese language (data for which bit pattern indicated on the display screen is previously determined) are transmitted as the code data and the code data having been transmitted is converted in the receiving side into the bit pattern for display on the screen before it is displayed. Therefore, in the receiving side, ROM (Read only Memory) for storing bit pattern of characters transmitted, CPU (Central Processing Unit) for retrieving characters stored and RAM (Random Access Memory) are required.
Accordingly, in order to receive the teletext signal with a television receiver not provided with a function to receive the teletext signal (receiver which can receive only the television broadcast signal), ROM, CPU, RAM and a circuit including a superimposing circuit to superimpose the teletext signal into the video signal (image signal) are required additionally. Various kinds of circuits having such function have been developed as the teletext signal receiving circuit.
Applied use of this teletext signal receiving circuit allows a personal computer to receive the teletext signal. Namely, a personal computer can be used as a new type information source. However, such a personal computer is not yet announced for the public. Described below is a PC (Personal Computer) card simply comprising the teletext receiving circuit explained above but such PC card does not yet exist actually.
FIG. 7 is a block diagram showing an example of structure of a conventional PC card comprising in direct the teletext signal receiving circuit explained above.
In this figure, a TV tuner unit 10 is composed of a tuner 10a and used as one of the peripheral circuits of a personal computer 30. The PC card 20 is composed of a teletext signal receiving circuit 200 (comprising a character data extracting circuit 20a, a decoding circuit 20b and a superimposing circuit 20c), a video decoder 20e, PC interface 20f and an A/D converter 20h and is loaded to an extension slot of the personal computer 30.
The personal computer 30 is composed of a card interface 30a, CPU 30b, ROM 30c, RAM 30d, VGA (Video Graphics Array) 30e, a keyboard interface 30f and a D/A converter 30g, etc. At its external side, a CRT (Cathode Ray Tube) display 40, a keyboard 41 and loud speakers 42 are connected.
In above structure, when an operator (viewer) operates the keyboard 41 to select the predetermined channel (broadcasting station), CPU 30b of the personal computer 30 obtains the data of broadcasting station selected by the operator via the keyboard interface 30f. CPU 30b supplies a control instruction to receive the information of the broadcasting station inputted by the operator to the TV tuner unit 10 and PC card 20 via the card interface 30a.
The TV tuner unit 10 receives the broadcast signal of the predetermined broadcasting station on the basis of the selection instruction supplied from the personal computer 30 and converts this broadcast signal into a composite video signal and a voice signal before these are outputted to the PC card 20.
The teletext signal receiving circuit 200 of the PC card 20 extracts the teletext signal superimposed on the composite video signal and outputs this teletext signal after the superimposing process as required with respect to the video signal. Namely, the character data extracting circuit 20a of the teletext signal receiving circuit 200 extracts character data included in the teletext signal superimposed in the vertical blanking period of the composite video signal and then supplies this character data to a decoding circuit 20b. The decoding circuit 20b retrieves, from ROM not illustrated, the bit pattern for display on the display screen corresponding to individual character data and outputs such bit pattern to the superimposing circuit 20c. The superimposing circuit 20c superimposes the bit pattern of characters outputted from the decoding circuit 20b and the composite video signal and outputs the superimposed signal, when the teletext signal selected by the operator is, for example, the superimposed dialog broadcast signal (broadcast signal where dialogs are displayed as character string at a part of the display screen) for the convenience of persons having handicap in the sense of hearing. Moreover, when the selected teletext signal is the ordinary teletext signal (broadcast signal consisting of only characters), the superimposing circuit 20c does not execute the superimposing process and outputs in direct the bit pattern transmitted from the decoding circuit 20b.
The decoding circuit 20b is also provided, as explained above, with CPU, ROM and RAM (not illustrated) for retrieving the bit pattern of characters corresponding to the code data transmitted from the transmitting side.
A selector 20d selects, when the broadcast selected by the operator is the teletext signal, an output signal of the superimposing circuit 20c and also selects, when the broadcast is the ordinary television signal, an output signal of the TV tuner unit 10 and outputs this signal to a video decoder 20e.
The video decoder 20e converts the input video signal (analog signal) to a digital signal through the A/D conversion and outputs this digital signal to a PC interface 20f as the digital signal of YUV 4:2:2 format (hereinafter referred to as the video data). The PC interface 20f supplies the input video data to the card interface 30a of the personal computer 30 via a buffer, etc. not illustrated.
An A/D converter 20h converts the input voice signal to digital data through the PCM system and supplies this digital data to the PC interface 20f as an audio data. The PC interface 20f supplies the input audio data to the card interface 30a of the personal computer 30 via the buffer, etc. not illustrated.
The video data inputted to the card interface 30a is supplied to VGA 30e via the bus for execution of the predetermined process and is then displayed on the CRT display 40. Moreover, the audio data is supplied to a D/A converter 30g via the card interface 30a for conversion into an analog audio signal. Thereafter this analog audio signal is supplied to loud speakers 42 and are then outputted as the audible voice data.
According to such example, the teletext signal can be received by means of the personal computer 30 utilizing an existing teletext signal receiving circuit 200.
As explained above, the decoding circuit 20b of the teletext signal receiving circuit 200 is provided with semiconductor devices such as ROM for storing the bit pattern for display on the screen, CPU for retrieving the bit pattern stored and RAM. These semiconductor devices are also mounted in the personal computer 30, resulting in the problem that the devices are duplicated and thereby the cost of the system as a whole becomes high and system size is also increased.
The character data extracted from the teletext signal is converted to the video signal in the PC card 20 and is then outputted. Therefore, the personal computer 30 has a problem that it cannot process the character data included in the teletext signal in the same manner as the character data (character code) processed therein.